Abstract

We report the discovery of 76 new T dwarfs from the UKIRT Infrared Deep
Sky Survey (UKIDSS) Large Area Survey (LAS). Near-infrared broad- and
narrow-band photometry and spectroscopy are presented for the new
objects, along with Wide-field Infrared Survey Explorer (WISE) and
warm-Spitzer photometry. Proper motions for 128 UKIDSS T dwarfs are
presented from a new two epoch LAS proper motion catalogue. We use these
motions to identify two new benchmark systems: LHS 6176AB, a T8p+M4 pair
and HD 118865AB, a T5.5+F8 pair. Using age constraints from the
primaries and evolutionary models to constrain the radii, we have
estimated their physical properties from their bolometric luminosity. We
compare the colours and properties of known benchmark T dwarfs to the
latest model atmospheres and draw two principal conclusions. First, it
appears that the H - [4.5] and J - W2 colours are more sensitive to
metallicity than has previously been recognized, such that differences
in metallicity may dominate over differences in Teff when
considering relative properties of cool objects using these colours.
Secondly, the previously noted apparent dominance of young objects in
the late-T dwarf sample is no longer apparent when using the new model
grids and the expanded sample of late-T dwarfs and benchmarks. This is
supported by the apparently similar distribution of late-T dwarfs and
earlier type T dwarfs on reduced proper motion diagrams that we present.
Finally, we present updated space densities for the late-T dwarfs, and
compare our values to simulation predictions and those from WISE.